This invention relates generally to machines and procedures for printing text or graphics on printing media such as paper, transparency stock, or other glossy media; and more particularly to incremental machines and methods that construct text or images from individual ink spots formed progressively on a printing medium, in a pixel arrayxe2x80x94as for example by a scanning inkjet printer, or most other forms of matrix printing. The invention is directed to mitigation of several kinds of printing artifacts.
(a) Uneven graininessxe2x80x94One class of overall image-quality problems in incremental printing may take the curious form of image regions in which local image quality is overly good. This can be a problem because local image quality can be so much better than nearby image quality that the contrast between the two becomes conspicuous in itself.
This phenomenon has two adverse consequences. First, two adjacent regions can be so different in image character as to present a sort of banded appearancexe2x80x94a band of generally acceptable quality set next to a band of strikingly fine quality. The difference simply destroys the illusion of looking at a unitary image, calling attention instead to some unknown peculiarities of the reproduction process.
Second, the higher quality in one region calls into question what would otherwise be considered acceptable quality in the other region. In other words, a xe2x80x9cgoodxe2x80x9d region and a xe2x80x9cbetterxe2x80x9d region add up to dissatisfaction with the good region.
In some incremental-printing technologies such appearances arise, in particular, very near the top and bottom ends of a page, because distinctly different modes of printing are applied in those two regions as compared with all the image portions between. This behavior is a result of the inability of some incremental technologies to print a swath that is partially on the printing medium and partially off.
If the edge of the sheet lies partway along an inkjet printhead, for instance, and the printhead scans along the edge, ink applied by the head is likely to be smeared erratically by upward curling of the edge of the sheet. In some cases it is also possible to damage the nozzles.
To avoid such effects, the end zones of the sheet are printed with the printhead or xe2x80x9cpenxe2x80x9d entirely on the sheet. Since several passes are needed to print an image in any region, such operation requires that the several printing passes be made in sequence, but with no print-medium advance relative to the printhead.
Pure printmask rotation of the sweep type is used, instead of advance-type rotation, to complete the image in these end zones. The process is taken up in the previously mentioned patent of Cleveland.
Generally such operation produces somewhat clearer or less-grainy image quality than the more-typical advance rotationxe2x80x94in which a corresponding xe2x80x9cnormalxe2x80x9d advance occurs between printing passesxe2x80x94employed in other parts of the sheet. This difference can give rise to the banding effects discussed above.
More generally, however, such banding effects can occur in any composite region that bridges subregions with and without normal advance. More generally still, such effects can occur in any printing process which uses distinctly different printing modes or techniques in immediately adjacent image regions.
Although elimination of these effects is highly desirable, the regions in which they appear are after all rather small, and literally peripheral. Hence it is extremely important that any methodology adopted to mitigate these artifacts pose a very minimal degree of disruption to the overall printing processxe2x80x94and also to the overall structure of the programming which controls that process.
(b) Better-known artifactsxe2x80x94Several other types of image-quality defects are well documented in the patent and other literature, and have been the subject of extensive corrective efforts. Yet as the modern competitive push continues toward ever higher photographic-quality images and ever faster but less expensive machines, such artifacts persist stubbornlyxe2x80x94and newer, more economical and less disruptive techniques are always at a premium.
Some such artifacts arise very directly from defects in nozzle geometry or firing characteristics. These first-order defects include white space and double-printingxe2x80x94due to nozzles that are not printing or are misdirected.
Other artifacts, harder to understand and still harder to uproot, come from unfortunate combinations of those simple nozzle-to-nozzle defects with regularity in the printing process. These include portions of the progressive migrating patterns which develop in error diffusion.
They can also include the repetitive stepping of printmasks that are not large enough to escape from the repetition-sensitive angular range of the eye, as taught in the previously mentioned patent document of Garcia. On the other hand, as Garcia has also pointed out, the opposite of excessive regularityxe2x80x94namely, excessive randomnessxe2x80x94can also lead to a different sort of artifact, namely undesirable visible granularity in an image.
Although all these defects can be managed very effectively by multipass printmode techniques and related tactics, those techniques and tactics in general levy a large price in terms of printing throughput. Such a penalty is increasingly less acceptable in the marketplace.
(c) Repetitive overprintingxe2x80x94Returning now to somewhat more-subtle difficulties, a problem that persists even in some sophisticated randomized printmasking schemes is repetitive overprinting of particular nozzles by specific other nozzles. In such situations the first-order problem of nozzle outages or misdirections is already eliminated by mixing inking by different nozzles in a single pixel rowxe2x80x94and even within an individual pixel.
This stratagem, however, may be to no avail if it happens that two or three nozzles used in conjunction, in a given row or pixel, all happen to be malfunctioning similarly or complementarily. Of course such a result is statistically less prevalent, but still significant.
(d) Software-generated beatsxe2x80x94Some repetitive visible effects originate in software-generated interferences. These include, for example, interactions between dither-mask and printmask periodicities as explained in the previously mentioned patent document of Borrell.
(e) Unrelated use of small advancesxe2x80x94For completeness it is mentioned here that it has been known heretofore to advance the printing medium, in addition to the normal advance used between printing passes, by supplemental very small amounts. These supplemental advancesxe2x80x94most typically many pixel rows, but in any event substantially greater than a single rowxe2x80x94are strictly for the purpose of compensating known errors in mechanical positioning, and have not been employed to address any of the problems discussed above.
(f) Conclusionxe2x80x94These several difficulties have continued to impede achievement of uniformly excellent inkjet printingxe2x80x94at high throughput. Thus important aspects of the technology used in the field of the invention remain amenable to useful refinement.
The present invention introduces such refinement. In its preferred embodiments, the present invention has several aspects or facets that can be used independently, although they are preferably employed together to optimize their benefits.
In preferred embodiments of a first of its facets or aspects, the invention is apparatus for printing desired images on a printing medium, by construction from individual marks formed in pixel row-and-column arrays. The apparatus includes a printhead mounted for scanning motion to form marks in a multiple-pixel-row swath on the printing medium.
The apparatus also includes a printing-medium advance mechanism providing relative motion between the printhead and printing medium. This relative motion is in a direction substantially orthogonal to the scanning motion.
A normal advance of the mechanism is equal to the height of at least several pixel rows; however, the apparatus also includes some means for stepping the printing medium a distance of roughly one pixel row or less. For purposes of breadth and generality in discussing the invention, these means will be called simply the xe2x80x9cstepping meansxe2x80x9d.
The foregoing may constitute a description or definition of the first facet of the invention in its broadest or most general form. Even in this general form, however, it can be seen that this aspect of the invention significantly mitigates the difficulties left unresolved in the art.
In particular, this first facet of the invention can print images in which graininess at the top and bottom ends of a page is about the same as in other regions. Although this aspect of the invention in its broad form thus represents a significant advance in the art, it is preferably practiced in conjunction with certain other features or characteristics that further enhance enjoyment of overall benefits.
For example, it is preferred that the stepping means include some means for stepping the distance with no corresponding data shift. Alternatively or in addition, it also preferred that the stepping means comprise means for stepping by said distance between printhead scans when there is no normal advance; in this case the stepping means roughly equalize graininess between image regions with and without normal advance.
Another preference is that the stepping means include some means for stepping by the above-mentioned distance between printhead scans adjacent to at least one end of a page. In this case preferably the stepping means do specifically equalize graininess between image regions adjacent to and remote from the end of a page.
Still another preference is that the purpose of the above-described advance introduced by the stepping means is to deliberately misalign successive swaths. (That is, the apparatus includes means for deliberately misaligning successive swaths, and these means comprise the stepping means.)
In yet another preference, the stepping means include some means for stepping distances equal to successively different fractions of a pixel row, respectively, between successive pairs of swaths. In this case preferably the successively different fractions are progressively decreasing.
In one particularly convenient decreasing sequence the successively different fractions approximately follow a cosine function. Preferably the cosine function is:       I    max    ⁢  cos  ⁢            I      ·      π              2      ⁢              I        max            
where I counts swaths printed without normal advances, and Imax is the largest value reached by I. In this situation preferably the stepping means comprise some means for initiating normal advances after I reaches Imax.
In an additional preference, still relative to the first major independent facet or aspect, the stepping means include some means for introducing a reciprocating motion into the positioning between the printhead and such printing medium. (In other words, the stepping is in the form of a reciprocation.) These means will be called, for reasons suggested earlier, the xe2x80x9cintroducing meansxe2x80x9d.
This preference importantly advances the art. In particular, this small-amplitude reciprocation has the effect of injecting spatial noise into the resulting image. This deliberately created disturbance helps to eradicate or mitigate visible artifacts that arise from regularity in the printing process.
For example, such artifacts can include white space or double-printing due to nozzles that are not printing or are misdirectedxe2x80x94or progressive migrating patterns such as developed in error diffusion. It can also include the repetitions of printmasks that are not large enough to escape from the repetition-sensitive angular range of the eye, as taught in the previously mentioned patent document of Garcia.
On the other hand, to the extent that the reciprocations introduced here are made random with respect to the printing process and image details, it is noteworthy that it is possible to inject excessive randomness. This too has been pointed out by Garcia, who shows that it can lead to undesirable visible granularity and that accordingly the ideal is a careful balance between excessive regularity and excessive randomness.
Within this reciprocation preference, certain additional subpreferences are significant. In particular, preferably the introducing means include means for providing the reciprocating motion in the form of a vibration.
In this case it is further preferred that the providing means comprise means for superimposing an oscillatory signal into electrical drive signals that control the scanning motionxe2x80x94or into electrical drive signals that control the printing-medium advance mechanism. Another basic preference is that the introducing means include some means for introducing the reciprocating motion in the form of successive offsets of substantially alternating direction.
In preferred forms of a second of its aspects, the invention is a method for printing desired images on a printing medium. The method operates by construction from individual marks formed in pixel row-and-column arrays, by a scanning printhead that operates in association with a printing-medium advance mechanism.
The method includes the steps of automatically operating the printing-medium advance mechanism under control of a programmed processor which follows this codexe2x80x94
The foregoing may constitute a description or definition of the second facet of the invention in its broadest or most general form. Even in this general form, however, it can be seen that this aspect of the invention too significantly mitigate the difficulties left unresolved in the art.
In particular, this form of the invention accomplishes imposition of a cosine function, such as described above, with a very minimal modification of otherwise conventional printer-control programming.
In preferred embodiments of third, fourth and fifth basic aspects or facets, the invention has certain elements in common with the first. More specifically, the invention is apparatus for printing desired images on a printing medium, by construction from individual marks formed in pixel row-and-column arrays.
As before the apparatus includes a printhead mounted for scanning motion to form marks in a multiple-pixel-row swath on the printing medium, and a printing-medium advance mechanism providing relative motion between the printhead and printing medium. Again this motion is in a direction substantially orthogonal to the scanning motion, and a normal advance of the mechanism is equal to the height of at least several pixel rows.
The third facet of the invention, however, expressly includes some means for roughly equalizing graininess between image regions with and without normal advance. These means, again for generality and breadth, will be called simply the xe2x80x9croughly equalizing meansxe2x80x9d. The roughly equalizing means in turn include some means for stepping a distance of roughly one pixel row or less between printhead scans when there is no normal advance.
The foregoing may represent a description or definition of the third aspect or facet of the invention in its broadest or most general form. Even as couched in these broad terms, however, it can be seen that this facet of the invention importantly advances the art.
In particular, this aspect of the invention makes possible printing of images in which graininess is equalizedxe2x80x94and therefore overall image quality is freer of undesirable artifactsxe2x80x94as between image regions with and without normal advance. Although the third major aspect of the invention thus significantly advances the art, nevertheless to optimize enjoyment of its benefits preferably the invention is practiced in conjunction with certain additional features or characteristics.
More specifically, the stepping means preferably comprise means for stepping by the indicated distance between printhead scans adjacent to at least one end of a page. In this case preferably the stepping means roughly equalize graininess between image regions adjacent to and remote from the end of a page.
Another preference is that the stepping means include some means for steppingxe2x80x94between successive pairs of swathsxe2x80x94distances respectively equal to successively different fractions of a pixel row. Here preferably the successively different fractions are progressively decreasing.
Still further preferably the successively different fractions approximately follow a cosine function. The cosine function in turn preferably is described by the following computer-program line.
paperAdvance=Amplitude*cos(Frequency*nPasses+Phase);
When these preferences are employed, preferably these definitions are used as well:
Amplitude is 7;
Frequency is 0.2244;
nPasses increments from zero through 7; and
Phase is zero.
In preferred embodiments of its fourth major independent facet or aspect, in addition to the common features mentioned just preceding discussion of the third aspect, the invention includes some means for stepping the printing-medium advance mechanism a distance of roughly one pixel row or less. This is done to deliberately misalign successive swaths between printhead scans remote from both ends of a page.
It will be noted that this aspect of the invention is thus counter to some preferences introduced above for the first and third aspects of the invention. This fourth facet of the invention accordingly is not aimed at mitigating top-of-page or bottom-of-page artifacts.
The foregoing may represent a description or definition of the fourth aspect or facet of the invention in its broadest or most general form. Even as couched in these broad terms, however, it can be seen that this facet of the invention importantly advances the art.
In particular, this aspect of the invention cures or mitigates the repetitive overprinting of particular nozzles by specific other nozzles. To the extent that some undesired regularity remains, in regard to such overprinting, the fourth facet of the invention perturbs that regularity by at least strongly affecting the frequency of the beats between different periodicities within the printer mechanism.
These mechanism periodicities include particularly, but not exclusively, software-generated beats. Such software-generated interferences include, for example interactions between dither-mask and printmask periodicities as explained in the previously mentioned patent document of Borrell.
Although the fourth major aspect of the invention thus significantly advances the art, nevertheless to optimize enjoyment of its benefits preferably the invention is practiced in conjunction with certain additional features or characteristics. In particular, several of the preferences mentioned above for the previously introduced aspects of the invention apply here as well.
All of the foregoing operational principles and advantages of the present invention will be more fully appreciated upon consideration of the following detailed description, with reference to the appended drawings, of which: